I tried to backup a database with SQL Backup and receive always the error message "Login failed for user 'XYZ'. Reason: The account is disabled."
Thats absolutely true, but we don't know where we have to change the user in SQL Backup.

The SQLBackup Service is running with a windows account and the SQL Agent as well. But if want to start a backup with SQL Backup it tells us another user that will be used to create the backup.

Is there any other location where I can change permission or the user which is used to create a backup of a database?

SQL Backup tries to impersonate the user doing the backup, so if you are connecting using SQL Backup Console and using Windows Authentication, all you should need to do is go into Active Directory and unlock your Windows domain account. If you connect using SQL Auth, I suspect the account management is done in SQL Server Management Studio somewhere.

SQL Backup doesn't have any facilities for managing accounts. (with one exception and that is when the SQL Backup Agent connects using SQL Authentication).

I'll tell you about how to configure SQL Backup Agent to use SQL authentication, but I'm not sure that will help. You mention you use SQL authentication to do backups - is this the sql account you specify in the connection properties to SQL Backup? If so, we are talking about two different things.